Hadrat r.a once said: Taqdeer in actual fact is the knowledge of Allah. In other words whatever Allah had planned before bringing the creation into existence; that a certain thing will be created at a certain time, it will remain alive for so many years and it will have such and such effects. This planning is called taqdeer. Thereafter whatever transpires in accordance with taqdeer is called qadha. The example of this is that of an architect who draws a plan of the house before building. According to the plan the house is built. The only difference between the two is that there is a possibility of a mistake occuring in the plan of the architect and upon realizing it work is sometimes done contrary to the plan. This is contrary to the the knowledge of Allah, in which there is no chance of any mistake occurring.